1. Field of the Invention
This invention concerns a device for the measurement of the viscoelasticity of products, and particularly those with a low viscosity.
2. Description of the Related Art
It is often necessary when measuring the physicochemical characteristics of a given product, such as a resin, a paint or a plastic, to measure and know the viscoelasticity of the product in question.
A great many devices for measuring viscoelasticity have been proposed. Some consist of two flat plates between which the product to be measured is deposited. An angular movement of one plate in relation to the other creates a shear torque in the product. A measurement of this shear torque serves to determine the viscoelasticity of the product deposited between the plates.
Other devices known under the generic designation of Couette type rheometers have a female cupel into which is put a male cupel, the cupels being cylindrical and the product to be measured being deposited in the female cupel. The product is sheared between the cupels, one of which is fixed and the other mobile.
The theory of rheometers of the Couette type and their structure are described in "Initiation a la Rheologie" (Introduction to Rheology), 1983 edition of "Technique et Documentation" (Lavoisier).
Other devices for measuring the viscoelasticity or rheometers are disclosed in FR-A-2,531,538, U.S. Pat. Nos. 3,667,286 and 2,828,621.
A serious drawback of all rheometers used until now resides in the fact that they operate in the open air. Because of this, when the measuring device is put into a heating cabinet or an oil bath to raise the product or substance to be measured to a designated temperature, which depends, among others, on the nature of the product to be measured, it follows that, in products containing a solvent, the solvent evaporates in more or less large quantities, and this occurs no matter how fast the product is put into the female cupel, or into the heating cabinet or oil bath. However, the evaporation of the solvent leads at least to a change in the physical characteristics of the product. Consequently, the shear torque is affected and this often leads to erroneous results.
Another serious drawback is related to the very structure of known cylindrical rheometers. Such rheometers, in addition to the fact that they are open and thus likely to permit evaporation of solvent, are relatively large in size and necessitate a change of the shear torque measuring sensor whenever there is a change to another product with a different viscosity. This results in an increased risk of damage to sensors which, furthermore, are very expensive. In addition, they are poorly suited to an alternating or sinusoidal movement between the mobile and fixed cupels due to the inevitable inertia caused by the oversizing of the cupels.